"One 4th Infantry Division soldier was killed and 12 were wounded during a rocket attack," on the military base near Balad, 75 kilometres (45 miles) north of the capital, a spokesman said.
Seven rockets were fired on the base at around 1:15pm (1015 GMT), another military spokesman said in Baghdad, leaving two of the wounded in a serious condition.
Helicopters returned fire and US forces arrested 12 men and four women after the attack.
An unexploded rocket was later found by US soldiers searching the area of the attack. They erected checkpoints as a security precaution.
The latest death brings to 251 the number of US soldiers killed in the 10-month insurgency battering Iraq since US President George Bush declared an end to major combat on 1 May 2003.
Separately, a US soldier died and two others were hurt when the army vehicle in which they were travelling crashed, said a statement from the US-led coalition.
The accident happened at about 11:00am near the town of Haditha 80 kilometres (50 miles) northwest of the western town of Ramadi.